how can i use external command subsitution as a filename about ffcast

[ropery]

Something like ffcast <ext_command> "$(date +%m-%d-%H:%M:%S)"?
Or if you mean to use a literal % in arguments to external commands, you need take into account format-string substitution & escape % as %%, e.g.: ffcast <ext_command> %%m-%%d

[shufuwa]

I wanted to add .png or .mkv at the end too

[ropery]

Like "$(date +%m-%d-%H:%M:%S).png"?

Honestly, still not sure what you're trying to do.
When asking for help, it's better to be specific & articulate your question / goal fully.

[shufuwa]

when i try this ffcast png "$(date +%m-%d-%H:%M:%S).png"
it says
:: saving to file: 03-14-16:04:45.png
[image2 @ 0x5598e6ed4140] Could not open file : 03-14-16:04:45.png
av_interleaved_write_frame(): Input/output error

i am in /tmp dir so i think there's no permission problem

[shufuwa]

my bad, i think its a unix thing